1. Field of the Invention
This invention relates to an adaptive equalizer for use in a digital line transmission system.
2. Background
Pulse transmission down telephone twisted pair cable is needed when speech is PCM encoded. For A-law PCM the basic information rate is 64K bits/sec; in digital local area telephony the need for additional bits for synchronization and signalling increases this to 80K bits/sec. For duplex operation on one pair of wires, a two to four wire hybrid technique can be used, in which case the line transmission rate remains at 80K bits/sec, or time separation can be used. In this case one or more PCM frames is assembled into one burst and sent out at a higher rate, usually of the order of three times the normal bit rate, with the wires conveying the bursts alternately in different direction.
Since the relatively high bit rates have to be transmitted over twisted pair cables optimized for audio band transmission, the pulses would be considerably attentuated and distorted at the end of the line. The waveform shown in FIG. 1a is an example showing the distortion to a 2 .mu.sec pulse after reception over 4 Km of 0.5 mm transmission cable. The waveform is amplified and shows that the leading edge is fairly "well behaved", but the trailing edge has a very slow decay, which can cause inter-symbol interference. For a continuous mode of transmission, where pulses of either polarity, or zero, may be transmitted consecutively, it is desirable to constrain the period of the pulses to within two periods, i.e. to be zero at the centre of the adjacent pulses and at a maximum at its centre.